gdt.h File Reference

The Global Descriptor Table (GDT). More...

#include "types.h"

Include dependency graph for gdt.h:

Classes
union	GDT::SegmentDescriptor
	Describes the structure of segment descriptors. More...
struct	GDT::Pointer
	Structure that describes a GDT Pointer (aka GDT Descriptor) More...
struct	GDT::SegmentDescriptor.__unnamed__
struct	GDT::SegmentDescriptor.__unnamed__
	Fields specific for Code Segment. More...
struct	GDT::SegmentDescriptor.__unnamed__
	Fields specific for Data Segment. More...
struct	GDT::SegmentDescriptor.__unnamed__
	Fields specific to the Task State Segment. More...
struct	GDT::SegmentDescriptor.__unnamed__
	Fields specific to the second part of the Task State Segment on x64. More...

Namespaces
	GDT
	Abstracts the GDT that, primarily, contains descriptors to memory segments.The GDT is a table that primarily contains segment descriptors. Segment descriptors has a size of 8 Bytes and contains the size, position, access rights, and purpose of such a segment. Unlike the LDT, the GDT is shared between all processes and may contain TSS and LDT descriptors. For the kernel, the first entry is required to be a null descriptor and the code and data segments. To support user-mode processes, additional TSS, code, and data segments for ring 3 must be added.

Enumerations
enum	Segments { SEGMENT_NULL = 0, SEGMENT_KERNEL_CODE, SEGMENT_KERNEL_DATA }
enum	GDT::Granularity { GDT::GRANULARITY_BYTES = 0, GDT::GRANULARITY_4KBLOCK = 1 }
	Unit of the segment limit. More...
enum	GDT::Type { GDT::TYPE_SYSTEM16 = 0, GDT::TYPE_SYSTEM32 = 1, GDT::TYPE_DATA = 2, GDT::TYPE_CODE = 3 }
	Segment type. More...
enum	GDT::Size { GDT::SIZE_16BIT = 0, GDT::SIZE_32BIT = 1 }
	Address width in code segment. More...
enum	GDT::Gate {   GATE_TSS = 0x1, GATE_LDT = 0x2, GDT::GATE_TSS_BUSY = 0x3, GATE_CALL = 0x4,   GDT::GATE_TASK = 0x5, GATE_INTERRUPT = 0x6, GATE_TRAP = 0x7 }
	gate type More...
enum	GDT::Mode { REAL_MODE, PROTECTED_MODE, LONG_MODE }
	Modus.

Detailed Description

The Global Descriptor Table (GDT).

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Class Documentation

GDT::SegmentDescriptor.__unnamed__

struct GDT::SegmentDescriptor.__unnamed__

Class Members
uint64_t	limit_low: 16	Least-significant bits of segment size (influenced by granularity!)
uint64_t	base_low: 24	Least-significant bits of base address.
uint64_t	type: 3	Meaning of those 3 bits depends on segment_type below.
enum Type	segment_type: 2	Segment type (influences the meaning of the 3 bits above)
uint64_t	privilege_level: 2	Ring for this segment.
bool	present: 1	Entry is valid iff set to 1.
uint64_t	limit_high: 4	Most-significant bits of segment size.
bool	available: 1	One bit to use for other purposes.
uint64_t	custom: 2	Meaning of those 2 bits depends on segment_type below.
Granularity	granularity: 1	Unit used as granularity for the segment limit.
uint64_t	base_high: 8	most-significant bits of base address

GDT::SegmentDescriptor.__unnamed__

struct GDT::SegmentDescriptor.__unnamed__

Fields specific for Code Segment.

See also

ISDMv3, 3.4.5.1; Code- and Data-Segment Descriptor Types

Class Members
uint64_t	__pad0__: 40	Ignored.
bool	code_accessed: 1	If set, the code segment was used since the last reset of this value.
bool	readable: 1	If set the code is readable (otherwise only executable)
bool	conforming: 1	If set, the execution of code from this segment is only allowed when running at a privilege of numerically less than or equal to privilege_level (i.e. the executor has the same or higher privileges). However, the executor's privileges remain unchanged. For nonconforming code segments (i.e., conforming is set to 0), execution is allowed only if the privileges are equal. Execution will cause a GPF in case of privilege violation.
uint64_t	__pad1__: 10	Ignored.
bool	is64bit: 1	64bit code segment Warning If set to 1, operation_size must be 0.
Size	operation_size: 1	Default address width.
uint64_t	__pad2__: 0	Remainder ignored.

GDT::SegmentDescriptor.__unnamed__

struct GDT::SegmentDescriptor.__unnamed__

Fields specific for Data Segment.

See also

ISDMv3, 3.4.5.1; Code- and Data-Segment Descriptor Types

Class Members
uint64_t	__pad0__: 40	Ignored.
bool	data_accessed: 1	If set, the data segment was used since the last reset of this value.
bool	writeable: 1	If set, data is writable (otherwise read only)
bool	expand_down: 1	Growing direction for dynamically growing segments.
uint64_t	__pad1__: 10	Ignored.
uint64_t	reserved: 1	Reserved, always set to 0!
Size	big: 1	Size of the stack pointer. Warning Has a different meaning in case expand_down is set to 1.
uint64_t	__pad2__: 0	Remainder ignored.

GDT::SegmentDescriptor.__unnamed__

struct GDT::SegmentDescriptor.__unnamed__

Fields specific to the Task State Segment.

See also

ISDMv3, 7.2.2; TSS Descriptor

Class Members
uint64_t	__pad0__: 40	Ignored.
enum Gate	gate: 3	TSS or LDT entry.
uint64_t	__pad1__: 0	Remainder ignored.

GDT::SegmentDescriptor.__unnamed__

struct GDT::SegmentDescriptor.__unnamed__

Fields specific to the second part of the Task State Segment on x64.

See also

ISDMv3, 7.2.3; TSS Descriptor in 64bit mode

Class Members
uint64_t	base64_high: 32	Most-significant part of 64bit TSS entries.
uint64_t	__pad0__: 8	Reserved.
uint64_t	zero: 5	Zero.
uint64_t	__pad1__: 0	Remainder ignored.